This add-on is not compatible with your version of Thunderbird because of the following:

Version 18.0.1
Released January 3, 2013
478.2 kB
Works with
Thunderbird 17.0 - 18.*

- Mac OS X is no longer supportedFixed:- menubar position when maximized and mail.tabs.drawInTitlebar is switched to false- toolbox overflow when not maximized and mail.tabs.drawInTitlebar is switched to false